Local installation fails on Linux (6.1 community, PHP 7.4): To enable extensions, verify that they a

I have the same problem like dbb in the German forum,
on Linux trying to install 6.1 community edition without Docker using composer install:

Several php extensions seem to be missing:

  To enable extensions, verify that they are enabled in your .ini files:  
    - /etc/php/7.4/cli/php.ini  
    - /etc/php/7.4/cli/conf.d/10-opcache.ini  
    - /etc/php/7.4/cli/conf.d/10-pdo.ini  
    - /etc/php/7.4/cli/conf.d/20-calendar.ini  
    - /etc/php/7.4/cli/conf.d/20-ctype.ini  
    - /etc/php/7.4/cli/conf.d/20-exif.ini  
    - /etc/php/7.4/cli/conf.d/20-ffi.ini  
    - /etc/php/7.4/cli/conf.d/20-fileinfo.ini  
    - /etc/php/7.4/cli/conf.d/20-ftp.ini  
    - /etc/php/7.4/cli/conf.d/20-gettext.ini  
    - /etc/php/7.4/cli/conf.d/20-iconv.ini  
    - /etc/php/7.4/cli/conf.d/20-json.ini  
    - /etc/php/7.4/cli/conf.d/20-mbstring.ini  
    - /etc/php/7.4/cli/conf.d/20-phar.ini  
    - /etc/php/7.4/cli/conf.d/20-posix.ini  
    - /etc/php/7.4/cli/conf.d/20-readline.ini  
    - /etc/php/7.4/cli/conf.d/20-shmop.ini  
    - /etc/php/7.4/cli/conf.d/20-sockets.ini  
    - /etc/php/7.4/cli/conf.d/20-sysvmsg.ini  
    - /etc/php/7.4/cli/conf.d/20-sysvsem.ini  
    - /etc/php/7.4/cli/conf.d/20-sysvshm.ini  
    - /etc/php/7.4/cli/conf.d/20-tokenizer.ini  

Most of these extensions are already installed and have active ini files ini `/etc/php/7.4/cli/conf.d`, like the ones mentioned in the error message. If I uncomment the lines `/etc/php/7.4/cli/php.ini` as well, I get a warning that they are activated twice ("already loaded in Unknown on line 0"). Do I have to add anything in the ini files in `conf.d`?

Trying to install via apt, like `sudo apt-get install php7.4-opcache` results in  
`0 upgraded, 0 newly installed`.

​What are we missing?